e1000e: enable EEE by default

Now that IEEE802.3az-2010 Energy Efficient Ethernet has been approved as
standard (September 2010) and the driver can enable and disable it via
ethtool, enable the feature by default on parts which support it.

Signed-off-by: Bruce Allan <bruce.w.allan@intel.com>
Tested-by: Jeff Pieper <jeffrey.e.pieper@intel.com>
Signed-off-by: Jeff Kirsher <jeffrey.t.kirsher@intel.com>